CPAP treatment of SDB involves the delivery of a pressurised breathable gas, usually air, to a patient's airways using a conduit and mask. Gas pressures employed for CPAP typically range from 4 cm H2O to 28 cm H2O, at flow rates of up to 180 L/min (measured at the mask), depending on patient requirements. The pressurised gas acts as a pneumatic splint for the patient's airway, preventing airway collapse, especially during the inspiratory phase of respiration.
CPAP apparatus comprises a flow generator or blower for supplying pressurised respiratory gas, such as air, to the patient via an air delivery tube leading to a patient interface, such as a nasal or oronasal mask, or nasal cushion or nasal pillows arrangement.
CPAP machines are known which incorporate humidifying devices, either separately from the flow generator or integrated therewith. An example of a flow generator/humidifier unit is the ResMed® S9 with H5i humidifier sold by the present Applicant. The volume of such humidifiers is relatively large due to the requirement of a large surface area to heat the water supply, for example an approximate volume of 1680000 mm3 (1,680 ml).
Humidification of the air supply is typically carried out by passing the air exiting the flow generator over the surface of a body of water in a heated water reservoir. However, such humidifiers are cumbersome and may be prone to spillage, and are relatively slow to activate and to adjust humidification level.